*The Reform Party, slated to haul in some $17 million in federal matching campaign funds, will almost certainly field a candidate. The party founder, businessman Ross Perot, hasn't yet signaled whether he'll run. But the party's first elected official, Minnesota Gov. Jesse Ventura, is urging former Republican Sen. Lowell Weicker to pursue the Reform nomination. (Senator Weicker was elected Connecticut governor on a third-party ticket in 1990.)

*Sen. Bob Smith of New Hampshire, who undocked with the GOP mother ship last week, may relaunch his presidential campaign under a third-party banner.

*Commentator Patrick Buchanan, a perennial Republican candidate, says he might have to consider a third-party run - maybe on the Reform ticket - if the GOP "establishment" doesn't give him a fair shake.

Third-party movements arise from two sources. They grow out of public frustration with the two major parties - as in, for example, the rise of the Reform Party. Or they spring from the dissent of a major-party faction that cannot endorse the nominee. That's the case with Senator Smith and Mr. Buchanan, who are put off by Texas Gov. George W. Bush's alleged hammerlock on the GOP nomination.

While third-party runs have never succeeded, they have played pivotal roles in determining the outcome of presidential races, as did Alabama Gov. George Wallace's 1968 candidacy and Mr. Perot's 1992 campaign.

So Republican and Democratic nominees ignore such candidacies at their peril. The possible candidates in 2000 could threaten both parties.

Weicker is liberal enough to siphon off votes from a Democratic nominee. But he also retains a small moderate-Republican following, especially in the Northeast, and could slice away a fraction of the GOP vote.

Both Smith and Buchanan could rally part of the "right-to-life" movement away from the GOP. In states like Massachusetts, Pennsylvania, and Michigan, that could also hurt a Democrat.

As things now stand, none of these three has any chance at winning. But their ability to win even a small percentage of the vote in key states could turn a close election.

Governor Bush may face the biggest challenge, assuming he is the Republican nominee. If he can't keep their partisans in the party fold, he may find Smith or Buchanan to be a thorn in his side right through Election Day.